1. Field of the Invention
This invention relates to circuits which extract nonrandom noise components from signals by charging a capacitor with a voltage representative of the noise and subtracting the voltage from the signal voltage.
2. Related Art
Certain electrical devices, and particularly image sensor arrays such as charge injection device (CID) arrays or HgCdTe sensor arrays, output voltages which include a signal component and a nonrandom, background noise component. The nonrandom background noise is due to leakage or a background signal.
More specifically, FIG. 1 depicts an electrical equivalent model of a typical sensor or detector which can be used with the present invention. The output voltage V.sub.1 of detector D is passed to a preamplifier stage. The preamplifier stage will be discussed below with regard to the invention.
The signal voltage V.sub.s ' from detector is generated in response to a change in voltage. Generally, there will be large (C.sub.large) and small (C.sub.small) capacitances associated with the detector D as shown, as well as leakage current I.sub.leakage which equals the noise current. Further, V.sub.1 is either a composite of a signal voltage V.sub.s and a nonrandom, background noise voltage signal V.sub.n (v.sub.noise), or V.sub.n alone. That is, ##EQU1## where T=the time interval for sampling either V.sub.n only or V.sub.s +V.sub.n, (see FIG. 2 timing diagram).
In FIG. 2, the voltage V.sub.2 is essentially V.sub.1 after preamplification and passage through a buffer stage (see below for further disussion). V.sub.2 includes V.sub.n. The slanted line or droop in V.sub.2 is V.sub.n due to I.sub.leakage. Note that V.sub.s =0 and V.sub.n =0 in this example.
In general, it is very difficult to capture V.sub.s at t=0 (see V.sub.2 in FIG. 2) due to the nonideal behavior of detector devices such as detector D, and electronic effects such as time delays and ringing. By the time V.sub.s is settled, V.sub.1 contains the unwanted noise voltage component V.sub.n.
In prior imaging systems, the noise problem has been approached by employing very accurate analog to digital convertors and filtering out the background noise which varies slowly compared to the signal. The need for high precision A/D convertors raises system requirements and system costs.
An effective means and method of extracting such nonrandom noise which eases system requirements is highly desirable, but heretofore undisclosed.